CODEWITHSUNDEEP
c++directx
Idris Khan
Idris Khan

Reputation: 73

How to update vertex buffer data frequently in DirectX 11?

I am trying to update my vertex buffer data with the map function in dx. Though it does update the data once, but if i iterate over it the model disappears. i am actually trying to manipulate vertices in real-time by user input and to do so i have to update the vertex buffer every frame while the vertex is selected.

Perhaps this happens because the Map function disables GPU access to the vertices until the Unmap function is called. So if the access is blocked every frame, it kind of makes sense for it to not be able render the mesh. However when i update the vertex every frame and then stop after sometime, theatrically the mesh should show up again, but it doesn't.

i know that the proper way to update data every frame is to use constant buffers, but manipulating vertices with constant buffers might not be a good idea. and i don't think that there is any other way to update the vertex data. i expect dynamic vertex buffers to be able to handle being updated every frame.

    D3D11_MAPPED_SUBRESOURCE mappedResource;
    ZeroMemory(&mappedResource, sizeof(D3D11_MAPPED_SUBRESOURCE));

    //  Disable GPU access to the vertex buffer data.
    pRenderer->GetDeviceContext()->Map(pVBuffer, 0, D3D11_MAP_WRITE_DISCARD, 0, &mappedResource);

    //  Update the vertex buffer here.
    memcpy((Vertex*)mappedResource.pData + index, pData, sizeof(Vertex));

    //  Reenable GPU access to the vertex buffer data.
    pRenderer->GetDeviceContext()->Unmap(pVBuffer, 0);

Upvotes: 2

Views: 3430

Answers (3)

Andrey
Andrey

Reputation: 6367

If you develop for UWP - use of map/unmap may result in sync problems. ID3D11DeviceContext methods are not thread safe: https://learn.microsoft.com/en-us/windows/win32/direct3d11/overviews-direct3d-11-render-multi-thread-intro.

If you update buffer from one thread and render from another - you may get different errors. In this case you must use some synchronization mechanism, such as critical sections. Example is here https://developernote.com/2015/11/synchronization-mechanism-in-directx-11-and-xaml-winrt-application/

Upvotes: 0

ErnieDingo
ErnieDingo

Reputation: 444

As this has been already answered the key issue that you are using Discard (which means you won't be able to retrieve the contents from the GPU), I thought I would add a little in terms of options.

The question I have is whether you require performance or the convenience of having the data in one location?

There are a few configurations you can try.

  1. Set up your Buffer to have both CPU Read and Write Access. This though mean you will be pushing and pulling your buffer up and down the bus. In the end, it also causes performance issues on the GPU such as blocking etc (waiting for the data to be moved back onto the GPU). I personally don't use this in my editor.
  2. If memory is not the issue, set up a copy of your buffer on CPU side, each frame map with Discard and block copy the data across. This is performant, but also memory intensive. You obviously have to manage the data partioning and indexing into this space. I don't use this, but I toyed with it, too much effort!
  3. You bite the bullet, you map to the buffer as per 2, and write each vertex object into the mapped buffer. I do this, and unless the buffer is freaking huge, I havent had issue with it in my own editor.
  4. Use the Computer shader to update the buffer, create a resource view and access view and pass the updates via a constant buffer. Bit of a Sledgehammer to crack a wallnut. And still doesn't stop the fact you may need pull the data back off the GPU ala as per item 1.

There are some variations on managing the buffer, such as interleaving you can play with also (2 copies, one on GPU while the other is being written to) which you can try also. There are some rather ornate mechanisms such as building the content of the buffer in another thread and then flagging the update.

At the end of the day, DX 11 doesn't offer the ability (someone might know better) to edit the data in GPU memory directly, there is alot shifting between CPU and GPU.

Good luck on which ever technique you choose.

Upvotes: 2

user7860670
user7860670

Reputation: 37468

Mapping buffer with D3D11_MAP_WRITE_DISCARD flag will cause entire buffer content to become invalid. You can not use it to update just a single vertex. Keep buffer on the CPU side instead and then update entire buffer on GPU side once per frame.

Upvotes: 1

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